Silicone pressure sensitive adhesives have excellent adhesive strength, tack, and cohesive strength, which are generally the properties required of a pressure sensitive adhesive. Silicone PSA tapes are used in many high temperature and chemically harsh industrial operations, such as plater's masking tapes, high shear splicing tapes, plasma and flame spraying, and the like. In order to be useful in these types of applications, silicone PSA's must exhibit high lap shear strength, particularly at elevated temperatures.
The term "pressure sensitive adhesives" (PSA) as used herein refers to adhesives that can be adhered to a surface and yet can be stripped from the surface without transferring more than trace quantities of adhesive to the surface, and can be re-adhered to the same or another surface because the adhesive retains some or all of its tack and adhesive strength.
A drawback associated with currently available silicone PSA's is the lack of high solids content with improved lap strength in combination with high peel and tack, especially at high temperatures. Presently there are no silicone PSA's which can demonstrate such adhesive properties without the presence of solvents. In geographic areas in which there are stringent air quality control regulations, the use of aromatic solvents in PSA's are subject to stringent regulation which effectively prohibits their use.
High solids silicone compositions capable of curing to form pressure sensitive adhesive compositions are known in the art, but these known compositions do not provide sufficient adhesive properties for demanding applications.
U.S. Pat. No. 5,169,727 (Boardman) is directed to a pressure sensitive adhesive composition having high solids content comprising (a ) a benzene soluble resinous copolymer consisting of R'R"R"'SiO.sub.1/2 units, SiO.sub.4/2 units and containing silicon-bonded hydroxyl radicals ranging from 1 to 4 percent by weight of the copolymer, (b) a diorganoalkenylsiloxy end-blocked polydiorganosiloxane, (c) a diorganohydrogensiloxy end-blocked polydiorganosiloxane, (d) a crosslinking agent, and (e) a hydrosilylation catalyst. Boardman requires an organosiloxane crosslinker having 1 to 15 silicone atoms. Further, the examples presented in Boardman show the pressure sensitive adhesives prepared had low to moderate tack properties. The adhesive strength ranged from poor to passing as measured by holding power at 70.degree. C. There is no teaching of a PSA with improved lap shear properties, no teaching of improved lap shear properties at high temperature, and no teaching of obtaining useful pressure sensitive adhesive properties at SiH/Si-Vinyl ratios of greater than 1.25:1.
U.S. Pat. No. 3,983,298 (Hahn et al.) is directed to a composition useful as a pressure sensitive adhesive, obtained by mixing components consisting essentially of (a) from 50 to 60 parts by weight of a solid, benzene soluble resin copolymer consisting essentially of R.sub.3 SiO.sub.1/2 units and SiO.sub.4/2 units, (b) from 40 to 50 parts by weight of an essentially cyclic free vinyl-terminated polydiorganosiloxane having a viscosity of from 20,000 to 100,000 centipoise at 25.degree. C., (c) a hydrogen-containing organopolysiloxane in an amount sufficient to provide from 1.0 to 20.0 silicon-bonded hydrogen atoms for every olefinically unsaturated radical in the total of (a) plus (b) and (d) a platinum catalyst It is pointed out in Hahn that compositions of the prior art containing MQ resins mixed with low viscosity polydiorganosiloxanes do not form PSA's.
U.S. Pat. No. 4,774,297 (Murakami et al..) is directed to a composition suitable for forming a pressure sensitive adhesive having excellent tack and adhesive strength comprising (A) 30-70 parts by weight of a vinyl-terminated polydiorganosiloxane having a viscosity of at least 500,000 centipoise at 25.degree. C., (B) 70-30 parts by weight of an organopolysiloxane containing R.sub.3 SiO.sub.1/2 units and SiO.sub.4/2 units, (C) an organohydrogensiloxane in an amount sufficient to provide 1-20 silicon-bonded hydrogen atoms per alkenyl group, (D) a platinum-containing catalyst, and (E) from 25-400 parts by weight of an organic solvent. In order to obtain a satisfactory product, Murakami et al.. teach that it is essential that the vinyl polymer have a viscosity of at least 500,000 centipoise and preferably at least one million centipoise at 25.degree. C.
U.S. Pat. No. 4,988,779 (Medford et al.) discloses a composition suitable for forming a pressure sensitive adhesive, the composition having a solvent content of no more than 5-10% by weight and comprising from 30 to 50 parts of a vinyl-containing polydiorganosiloxane fluid having a viscosity of from 500 to 10,000 centipoise at 25.degree. C., from 50 to 70 parts of a benzene soluble resin copolymer containing R.sub.3 SiO.sub.1/2 units and SiO.sub.4/2 units, an organopolysiloxane having silicon bonded hydrogen atoms, and a platinum catalyst The hydrogen-containing organopolysiloxane of the formula R.sup.3.sub.a H.sub.b SiO.sub.(4-a-b)/2, is present in an amount sufficient to provide from 1.0 to 30.0 silicon-bonded hydrogen atoms for every olefinically unsaturated radical in the composition. The hydrogen-containing organopolysiloxane functions as a cross-linker and has a small structure with a=1.00 to less than 2.00, b=0.05-1.00, a+b=1.10 to less than 3.00. There is no teaching of using higher molecular weight hydride crosslinker to give better peel and tack properties. The peel adhesion of the cured adhesive was controlled by controlling the amount of MQ resin and not by the crosslinker. There is no teaching of obtaining high lap shear strength in combination with high peel and tack adhesive properties.
U.S. Pat. No. 5,190,827 (Lin) is directed to a composition having high solids content and having a hydrogen-containing polydiorganosiloxane containing more than 2 silicon-bonded hydrogen atoms per chain. Other components in the composition include an MQ resin, an alkenyl-terminated silicone fluid, and a hydride-terminated silicone fluid. The cross linking that occurs is only through the hydride crosslinker which is selected from linear or resinous siloxane polymers, and no organosiloxane containing more than 2 silicon-bonded alkenyl groups as a crosslinker is disclosed.
U.S. Pat. No. 5,292,586 (Lin et al.) discloses a composition comprising a silanol-containing MQ resin, an alkenyl-terminated polydiorganosiloxane, a hydride-terminated organohydrogenpolysiloxane and a catalytic amount of a hydrosilylation catalyst. The composition cures to form a PSA having high peel and tack adhesion properties, but does not contain any cross linker or multifunctional alkenyl or multifunctional hydride silicones. The terminal hydride adhesive network reacts with silanol of the MQ resin to form an internally cured adhesive network.
U.S. patent application Ser. No. 08/150,570 (now abandoned) discloses an addition cured adhesive composition having high solids content. The composition is prepared from a multifunctional vinyl-containing silicone as crosslinker, in addition to MQ resin, an alkenyl-terminated silicone fluid, a hydride-terminated silicone fluid and a hydrosilylation catalyst. The cross-linking occurs through the external vinyl-crosslinker and forms the cured pressure sensitive adhesive. There is no teaching of preparing PSA's with improved lap shear strength in combination with high peel and tack adhesive properties.
While it has continued to be desirable to provide high solids polydiorganosiloxane compositions which cure to yield silicone pressure sensitive adhesives having high solids content and improved lap shear properties, particularly at elevated temperatures, low temperature uses that do not activate the curing reaction have been foreclosed because the conventional addition cure formulations do not cure well at low temperatures.
Pressure sensitive adhesives cure by hydrosilation of an olefinically unsaturated siloxane with a hydrogen containing siloxane, or by a free radical initiated polymerization, or by a combination of the two. Thus the pressure sensitive adhesives of the prior art typically contain a curing agent of some type either a noble metal hydrosilylation catalyst or a peroxide free radical generating compound. Pressure sensitive adhesives that cure by a combination cure will contain both types of catalysts.